The present invention relates to a vapor-phase growth method for a nitride semiconductor, which can be adapted to form a nitride semiconductor, such as a gallium nitride based compound semiconductor, on a base body by vapor-phase growth, and to a nitride semiconductor device, such as a nitride semiconductor light emitting diode, a nitride semiconductor laser, a suitable nitride semiconductor electron device formed by using the vapor-phase growth method or the like. More specifically, the present invention relates to a vapor-phase growth method for a nitride semiconductor, which can be adapted to form an anti-surfactant film on a base body and form a nitride semiconductor layer by crystal growth from an opening portion formed in the anti-surfactant film, and a nitride semiconductor device formed by using the vapor-phase growth method.
In recent years, nitride based III–V compound semiconductors, such as GaN, AlGaN and GaInN, have become a focus of attention. This is because such a semiconductor has a forbidden band width ranging from 1.8 eV to 6.2 eV, and therefore, the semiconductor theoretically allows realization of a light emitting device enabling emission of light ranging from red light to ultraviolet light.
In the case of fabricating a light emitting diode (LED) or a semiconductor laser by using a nitride based III–V compound semiconductor, it is required to form a structure in which multi-layers, such as a GaN layer, an AlGaN layer, a GaInN layer, and the like, are stacked such that a light emitting layer (active layer) is sandwiched between an n-type cladding layer and a p-type cladding layer. In some cases, such a light emitting diode or a semiconductor laser includes a light emitting layer having a quantum well structure composed of GaInN/GaN or GaInN/AlGaN.
A vapor-phase growth technique for a nitride semiconductor, such as a gallium nitride based compound semiconductor, can be problematic because it can be difficult to obtain a substrate that is lattice matched with a nitride semiconductor or a substrate having a low density of dislocations. To solve such a problem, there is known a technique of depositing a low temperature buffer layer made from Aln or AlxGa1-xN (0≦x<1) at a low temperature of 900° C. or less on a surface of a substrate made from sapphire or the like, and then growing a gallium nitride based compound semiconductor thereon, thereby reducing dislocations due to lattice mismatching between the substrate and the compound semiconductor. Such a technique has been disclosed, for example, in Japanese Patent Laid-open No. Sho 63-188938 and Japanese Patent Publication No. Hei 8-8217. By using such a technique, it is possible to obtain a gallium nitride based compound semiconductor with improved crystallinity and morphology.
Another technique of obtaining a high quality crystal structure at a low density of dislocations has been disclosed, for example, in Japanese Patent Laid-open Nos. Hei 10-312971 and Hei 11-251253. This method involves depositing a first gallium nitride based compound semiconductor layer, forming a protective film made from a material capable of inhibiting growth of a gallium nitride based compound semiconductor, such as silicon oxide or silicon nitride, in such a manner as to selectively cover the first gallium nitride based compound semiconductor, and growing a second gallium nitride based compound semiconductor in an in-plane direction (lateral direction) from regions, not covered with the protective film, of the first gallium nitride based compound nitride layer, thereby preventing propagation of through-dislocations extending in the direction perpendicular to the interface of the substrate.
A further technique of reducing a density of through-dislocations has been disclosed, for example, in a document (MRS Internet J. Nitride Semicond. Res. 4S1, G3. 38 (1999)). This method involves growing a first gallium nitride based compound semiconductor, selectively removing the thus formed semiconductor film by using a reactive ion etching (hereinafter, referred to as “RIE”) system, and selectively growing a second gallium nitride based compound semiconductor from the remaining crystal in the growth apparatus. According to this method, it is possible to obtain a crystal film having a density of dislocations, which is reduced to about 106/cm2, and hence to realize a high life semiconductor laser using the crystal film formed according to this method.
By the way, in the case of growing a gallium nitride based compound semiconductor layer by such a selective growth technique, a selective growth portion has a three-dimensional structure called “facet” having a tilt plane which is a stable plane growing at a low crystal growth. For example, in the case of forming an underlying layer on a sapphire substrate with a C-plane of sapphire taken as a principal plane of the substrate, covering the surface of the underlying layer with a silicon oxide film as an anti-surfactant film (growth-inhibiting film), and selectively growing a gallium nitride layer from an opening portion provided in the silicon oxide film by supplying a source gas, crystal growth portion has a pyramid shape, for example, a hexagonal pyramid shape having a tilt plane covered with a crystal plane, such as an S-plane.
In this case, however, since a crystal growth rate is generally low at the tilt plane, a supplied source of a group III element is not deposited but migrated at the tilt plane. On the contrary, at a top portion of the pyramid shaped crystal growth layer, since the supplied amount of the source becomes excessively large, the crystal growth rate becomes significantly high. As a result, the top portion of the crystal growth layer contains a number of defects such as point defects, that is, has poor crystallinity. Further, crystal growth at the top portion of the crystal growth layer does not result in a smooth surface, and accordingly, in the case of fabricating a semiconductor device having an active layer or a pn-function particularly on the top portion, performances of the semiconductor device are significantly degraded.
A need, therefore, exists to provide improved nitride semiconductors that can be readily made and effectively used in a variety of applications.